The completion of the Tibet railway is being hailed in China as one of the world's great engineering marvels.

The longest high-altitude railway in the world, it will ease access to the remote region. Test runs are due to begin on 1 July.

Tibet's extraordinary isolation has kept it poor. Education levels and life expectancy fall well behind the rest of China. But that isolation has also helped to preserve Tibet's unique culture and way of life.

The arrival of the railway will bring tremendous change. China's communist rulers say it will open up Tibet, bringing greater prosperity for its entire people. Detractors say the opening of the railway is the death knell of an independent Tibetan culture.

It doesn't feel like our home any more

Sedeng Tibetan shop owner

Before the railway there were only two ways into Lhasa: an expensive plane ride, followed by a hair-raising touch down; or three days and nights on an overcrowded bus bouncing along back-breaking mountain roads.

Many a bus, and its passengers, has ended up crushed at the bottom of a lonely ravine.

Along the route of the railway, opinions vary about whether it is good or bad. While visiting a remote construction site on my most recent trip to Tibet I came across two Tibetan herders lounging beside the road. They were sitting on top of huge bundles of yak wool.

What did they think of the railway?

"It's a good idea" they said. "It'll make it easier for us to take our wool down to the market. At the moment we have to hire a truck to come up here, but with a train it'll be cheaper and easier."

Environmentalists worry about what effect the railway will have on the migration routes of rare Tibetan antelope, and about a fragile ecosystem that, once damaged, will take generations to repair.

But the real controversy is over what the railway will do to places like Lhasa.

When Francis Younghusband forced his way in to Lhasa at the head of an invading British army 100 years ago, he found a medieval city untouched by the outside world.

Today the Dalai Lama lives in exile in India. Many of the great lamaseries lie in ruins. Some have been rebuilt in the last few years, but these days the monks number in the hundreds. Often the inhabitants of these once-great religious centres are outnumbered by camera clicking tour groups.

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In the heart of Lhasa the ancient streets of the Barkor district still throng with groups of Tibetan pilgrims. Wizened old ladies with long grey plaits and sun-crinkled faces prostrate themselves every three steps as they process around the glorious Jokang temple, the oldest and most sacred in Tibet. Here the richness of Tibetan culture seems as deep and alive as ever.

But a few streets away you suddenly run into a completely different Lhasa. It is a city of broad straight avenues, flanked by white-tiled department stores. This city is in every way Chinese, down to the tacky plastic palm trees - the signature of a Chinese city-planning bureaucrat.

Here the people are also Chinese. The accents and dialects are of Sichuan and Hubei, Anhui and Henan - overcrowded provinces far away in China's east.

People from these poor rural provinces are flocking to Tibet, drawn by the pull of money. Beijing is pumping tens of billions of dollars in to Tibet. Lhasa is a boom town; construction sites abound. But I searched in vain for a Tibetan worker.

'Better at business'

In a small backstreet of the Barkor I found a Tibetan art gallery. Rows of young men were bent over easels hand-painting beautiful Tibetan tankas - religious paintings depicting scenes of a Buddhist heaven.

In the corner of the shop I found its owner, a handsome Tibetan man with a winning smile. I asked Sedeng how he felt about what was happening to Lhasa.

Tourists outnumber monks in some parts of Lhasa

"Of course we don't like it," he said. "The new city just looks like any other in the rest of China. It doesn't look like anything Tibetan. It doesn't feel like our home any more."

Other Tibetans I met complained about the large influx of migrants from eastern China.

"They are much better at doing business than we are," one taxi driver told me. "They have lots of connections in other parts of China, and they can always get products cheaper than we can, so it's impossible to compete."

I searched in my own mind for an analogy for what is happening in Tibet. It is similar in many ways to what happened to Native American tribes in the 19th century.

As the railroads opened the west of the United States, they brought with them a huge influx of new population. The native peoples were pushed to the margins, dumped on reservations, their traditional way of life left in ruins.

As the trainloads of migrants begin to roll in to Lhasa station, does Tibet face a similar future?